The present invention relates to a holographic recording apparatus and a method for recording holograms.
The development of holographic recording and reproducing apparatuses for recording data employing holography has been taking place.
Such a holographic recording and reproducing apparatus is capable of generating two types of beams, a modulated signal beam (with multiplexed data) and a non-modulated reference beam. These beams are emitted at the same point on a holographic recording medium. As a result, the signal beam and the reference beam interfere with each other to form a diffraction grating (hologram) at the incident point, and data is recorded on the holographic recording medium.
Irradiating a recorded holographic recording medium with a reference beam causes a diffractive beam (reproduction beam) to be generated from the diffraction grating formed on the holographic recording medium when data was recorded. Since the reproduction beam includes data multiplexed in the signal beam used for recording, the signal beam used for recording can be reproduced by receiving the reproduction beams with a light receiving element.
Known holographic recording and reproducing apparatuses are not capable of directly modulating the laser beam when a solid-state laser or a gas laser is used as a laser source. Therefore, known holographic recording and reproducing apparatuses are provided with a modulating device, such as a mechanical shutter or an acousto-optical modulator (AOM), for modulating the laser beam after it leaves the light source. When a modulating device is used, it is difficult to reduce the size of the optical system of the holographic recording and reproducing apparatus and to reduce the production cost of such a holographic recording and reproducing apparatus.
For such a holographic recording and reproducing apparatus, the above-mentioned problems can be solved by using a laser diode. To improve the coherence of the laser diode, the use of an external resonator laser has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 11-107377, paragraphs 0028 to 0031 and FIG. 1). When a laser diode is used as a light source, the laser beam can be directly modulated, and, thus, a laser beam modulating device for modulating a beam emitted for recording does not have to be provided.
When a laser diode is directly modulated, noise is generated at the rising edge of the beam due to the electrical current being modulated, affecting the holographic recording.
When using an external resonator laser, the intensity of the generated beam is unstable due to relaxation oscillation and some time is required until the wavelength of the beam stabilizes. If holographic recording is carried out while the wavelength is unstable (i.e., while a plurality of wavelengths are generated) and the wavelengths included in one pulse changes, the emission angle of the reproduction beam emitted from the hologram recorded with the two difference wavelength will not be aligned and noise will be reproduced.